Vagal stimulation increases collateral resistance and prolongs collateral time constants. The effect is greatest with segment hypocapnia and least with normocapnia. Atropine decreases collateral time constants and resistance. Clearly therefore collateral pathways receive parasympathetic innervation. There have been no studies of adrenergic effects on collaterals. In airways, beta blockade enhances the effect of bronchoconstrictors. We therefore propose to measure collateral resistance and time constants before and after beta blockade with propranolol and to investigate the interaction of local CO2 levels and vagal stimulation before and after beta blockade. We propose to measure collateral resistance and time constants in sensitized ponies before and after aerosol challenge with ovalbumin which cause a biphasic pulmonary response. Cooling of the surgically relocated vagus nerve will allow investigation of the role of the parasympathetic system in any observed resistance changes. We will investigate the local and central effects of CO2 in collateral resistance. Collateral resistance will be measured in the left lower lobe with blood flow occluded allowing control of local CO2 levels. Central CO2 levels will be controlled by varying gas tensions in the right perfused lung.